Pneumatic actuating drives are known which have a position regulator in order to regulate a desired opening level of a fitting, which is connected to the pneumatic actuating drive, on the basis of a nominal value preset. By way of example, the fitting may be a process valve within a pipeline system of a process installation. However, pneumatic actuating drives can also be suitable for operation of other industrial fittings.
The product prospectus “Der kompakte, intelligente Stellungsregler” [The compact, intelligent position regulator] (ABB Automation Products GmbH, document number: 50/18-19 DE RevA: June 2005 edition) discloses an electronic position regulator for a pneumatic actuating drive. The position regulator is formed as a type of electronics box which can be fitted to the pneumatic actuating drive. The position regulator is a configurable appliance which can communicate, for example, by a field bus. An exemplary functional aspect is a microprocessor-controlled procedure for a regulation program with a sampling rate of 20 ms. The nominal value can, in this case, be preset via a field bus connection which is designed using two-conductor technology.
Furthermore, the position regulator can have a supply air connection for an air pressure up to a maximum of 6 bar, as well as a working connection for passing on the control pressure generated by the position regulator to the control chamber of the pneumatic position regulator. A sensor input can be provided for an actual value of the present position of the switching element which is operated by the pneumatic actuating drive, which position is obtained via a position sensor arranged on the switching element. The pneumatic drive for the actuating drive can be provided continuously by an I/P module with a downstream 3/3-way valve. The 3/3-way valve controls the passage for ventilation or venting of the actuating drive as proportionally as possible. The configuration and observation of the operating state of the position regulator can be carried out either by a built-in control panel directly in situ, or centrally via a communication connection, on the basis of the bus protocol by a superordinate control unit.
US 2007/0045579 A1 discloses a pneumatic position regulator which is formed as an I/P module with a 3/3-way valve. The 3/3 switching function provides for switch positions of ventilation, closed position and venting of a working connection, which supplies control pressure for the connected pneumatic actuating drive. The electropneumatic valve with a 3/3 switching function can have two closure elements, which point in mutually opposite directions of the actuating movement and act with a same magnitude with respect to one another, each of which bound an internal control chamber, with a common control pressure connection being associated with the two control chambers. While one closure element can be used for ventilation of the working connection, the other closure element can be used for venting of the working connection. When neither of the two closure elements is operated, then the valve is in a closed position.
For position regulation, an electropneumatic valve such as this is intended to provide as proportional a response as possible for the electrical drive signal with respect to the pneumatic manipulated variable, with disturbance variables, which are caused inter alia by the forces on the switching element of the fitting and the hysteresis, reacting on the pneumatic side. Furthermore, influencing variables such as temperature fluctuations, pressure fluctuations and the like, can disturb an ideal proportionality ratio. In order to achieve a response which is as linear as possible, the regulation in some cases takes account of correction values determined using sensors. The results which can be achieved in this way are, however, not necessarily satisfactory. Another influencing factor is hysteresis of the valve mechanism. In order to achieve high regulation accuracy, it is desirable to gain knowledge relating to the amount by which the actuating signal should be varied to overcome the hysteresis, such that the switching element is moved in a desired manner.